地膜覆盖的玉米地的土壤空气状况

The effects of plastic mulching on soil aeration at the soil depth of 0-100 cm were studied in a corn field. The results indicated that the CO₂ concentration of unmulched soil in the 0-100 cm layer ranged from 0.001 to 0.016 m³/m^3, and that of mulched soil 0.002 to 0.018m³/m³, about 32.39% higher than the former on the average. Such a CO₂ concentration in the soil air is still suitable for crop growth. The O₂ concentration was inversely correlated with CO₂ concentration in the soil air (unmulching r=-0.92^**, mulching r=-0.79^*). O₂ concentration ranged from 0.11 to 0.17 m³/m³ in the mulched soil and 0.13 to 0.18 m³/m³ in the unmulched soil. By contrast, N₂ concentration in soil air remained relatively steady, with no difference between the two treatments. The relationship between the soil respiratory intensity and the depth of a soil layer appeared to be a power function. At the layer of 0-20 cm, the soil respiration intensity in the mulched soil was obviously higher than that in the unmulched. Plastic mulching could also affect soil structure. In comparison with the unmulched soil, the content of >0.25 mm aggregate and 0.05-0.001 mm microaggregate in the mulched soil was reduced by 82.1% and 35.8%, respectively; the soil total porosity, gaseous phase rate and aeration porosity in the depth of 10-20 cm were reduced by 2.85%, 19.89% and 26.54% respectively, but contrary at the depth of 0-10 cm.     

Carbon mineralization in subtropical alluvial arable soils amended with sugarcane bagasse and rice husk biochars

Subtropical recent alluvial soils are low in organic carbon (C). Thus, increasing organic C is a major challenge to sustain soil fertility. Biochar amendment could be an option as biochar is a C-rich pyrolyzed material, which is slowly decomposed in soil. We investigated C mineralization (CO₂-C evolution) in two types of soils (recent and old alluvial soils) amended with two feedstocks (sugarcane bagasse and rice husk) (1%, weight/weight), as well as their biochars and aged biochars under a controlled environment (25 ±2 ℃) over 85 d. For the recent alluvial soil (charland soil), the highest absolute cumulative CO₂-C evolution was observed in the sugarcane bagasse treatment (1 140 mg CO₂-C kg^-1 soil) followed by the rice husk treatment (1 090 mg CO₂-C kg^-1 soil); the lowest amount (150 mg CO₂-C kg^-1 soil) was observed in the aged rice husk biochar treatment. Similarly, for the old alluvial soil (farmland soil), the highest absolute cumulative CO₂-C evolution (1 290 mg CO₂-C kg^-1 soil) was observed in the sugarcane bagasse treatment and then in the rice husk treatment (1 270 mg CO₂-C kg^-1 soil); the lowest amount (200 mg CO₂-C kg^-1 soil) was in the aged rice husk biochar treatment. Aged sugarcane bagasse and rice husk biochar treatments reduced absolute cumulative CO₂-C evolution by 10% and 36%, respectively, compared with unamended recent alluvial soil, and by 10% and 18%, respectively, compared with unamended old alluvial soil. Both absolute and normalized C mineralization were similar between the sugarcane bagasse and rice husk treatments, between the biochar treatments, and between the aged biochar treatments. In both soils, the feedstock treatments resulted in the highest cumulative CO₂-C evolution, followed by the biochar treatments and then the aged biochar treatments. The absolute and normalized CO₂-C evolution and the mineralization rate constant of the stable C pool (K_s) were lower in the recent alluvial soil compared with those in the old alluvial soil. The biochars and aged biochars had a negative priming effect in both soils, but the effect was more prominent in the recent alluvial soil. These results would have good implications for improving organic matter content in organic C-poor alluvial soils.     

玉米秸秆应用和腐化对钙质苏打土改良的影响: 实验室试验研究

A laboratory lysimeter experiment was conducted to investigate the effects of forage corn (Zea mays L.) stalk application on the CO₂ concentration in soil air and calcareous sodic soil reclamation. The experimental treatments tested were soil exchangeable sodium percentage (ESP) levels of 1, 11, and 19, added corn stalk contents of 0 to 36 g kg^-1, and incubation durations of 30 and 60 days. The experimental results indicated that corn stalk application and incubation significantly increased CO₂ partial pressure in soil profile and lowered pH value in soil solution, subsequently increased native CaCO₃ mineral dissolution and electrolyte concentration of soil solution, and finally significantly contributed to reduction on soil sodicity level. The reclamation effciency of calcareous sodic soils increased with the added corn stalk. When corn stalks were added at the rates of 22 and 34 g kg^-1 into the soil with initial ESP of 19, its ESP value was decreased by 56% and 78%, respectively, after incubation of 60 days and the leaching of 6.5 pore volumes (about 48 L of percolation water) with distilled water. Therefore, crop stalk application and incubation could be used as a choice to reclaim moderate calcareous sodic soils or as a supplement of phytoremediation to improve reclamation effciency.     

间伐对杉木林土壤CO2通量的影响

Forest management is expected to influence soil CO₂ efflux (FCO₂) as a result of changes in microenvironmental conditions, soil microclimate, and root dynamics. Soil FCO₂ rate was measured during the growing season of 2006 in both thinning and non-thinning locations within stands ranging from 0 to 8 years after the most recent thinning in Chinese fir (Cunninghamia lanceolata (Lamb.) Hook) plantations in Huitong Ecosystem Research Station, Hunan, China. Soil temperature and moisture were also measured to examine relationships between FCO₂ and soil properties. Forest thinning resulted in huge changes in FCO₂ that varied with time since cutting. Immediately following harvest (year 0) FCO₂ in thinning area increased by about 30%, declined to 20%-27% below pre-cutting levels during years 4-6, and recovered to pre-cutting levels at 8 years post-cutting. A similar temporal pattern, but with smaller changes, was found in non-thinning locations. The initial increase in FCO₂ could be attributed to a combination of root decay, soil disturbance, and increased soil temperature in gaps, while the subsequent decrease and recovery to the death and gradual regrowth of active roots. Strong effects of soil temperature and soil water content on FCO₂ were found. Forest thinning mainly influenced FCO₂ through changes in tree root respiration, and the net result was a decrease in integrated FCO₂ flux through the entire felling cycle.     

间伐和去除凋落物对马尾松林细根生产力和土壤有机碳含量的影响

Soils play a critical role in the global carbon cycle, and can be major source or sink of CO₂ depending upon land use, vegetation type and soil management practices. Fine roots are important component of a forest ecosystem in terms of water and nutrient uptake. In this study the effects of thinning and litter fall removal on fine root production and soil organic carbon content were examined in 20-year-old Masson pine (Pinus resinosa) plantations in Huitong, Hunan Province of China in the growing seasons of 2004 and 2005. The results showed that fine root production was significantly lower in the thinning plots than in the control plots, with a decrease of 58% and 14% in 2004 and 2005 growing seasons, respectively. Litter fall removal significantly increased fine root production by 14% in 2004. Soil temperature (T_soil) and soil moisture (M_soil) were higher in the thinning plots than those in the controls. Litter fall removal had significant effiects on T_soil and M_soil. Soil organic carbon content was higher in the thinning plots but was lower in the plots with litter fall removal compared with that in the controls. Our results also indicated that annual production of fine roots resulted in small carbon accumulation in the upper layers of the soil, and removal of tree by thinning resulted in a significant increase of carbon storage in Masson pine plantations.     

Soil Erosion as A ffected by Polyacrylamide Application Under Simulated Furrow Irrigation with Saline Water

The reduction of soil and water losses under furrow irrigation with saline water is important to environmental protection and agricultural production.The objective of this study was to determine the effect of polyacrylamide(PAM)application on soil infiltration and erosion under simulated furrow irrigation with saline water.Polyacrylamide was applied by dissolving it in irrigation water at the rates of 1.5,7.5,and 15.0 mg L^-1 or spreading it as a powder on soil surface at the rates of 0.3,1.5,3.0,and 6.0 g m^-2,respectively.The effectrolyte concentration of tested irrigation water was 10 and 30 mmolc L^-1 and its sodium adsorption ratio(SAR)was 0.5,10.0,and 20.0(mmol_c L^-1)_0.5.Distilled water was used as a control for irrigation water quality.Results indicated that the effectrolyte concentration and SAR generally did not significantly affect soil and water losses after PAM application.Infiltration rate and total infiltration volume decreased with the increase of PAM application rate.Polyacrylamide application in both methods significantly reduced soil erosion,but PAM application rate did not significantly affect it.The solution PAM application was more effective in controlling soil erosion than the powdered PAM application,but the former exerted a greater adverse influence on soil infiltration than the latter.Under the same total amounts,the powdered PAM application resulted in a 38.2%-139.6% greater infiltration volume but a soil mass loss of 1.3-3.4 times greater than the solution PAM application.     

Zokor activity promotes soil water infiltration in the Mu Us sandy land of northern Shaanxi, China

Zokors are common subterranean rodents that inhabit agricultural fields, shrublands, and grasslands in the arid and semi-arid regions of China. Zokor burrowing activities can alter soil structure and affect soil hydrological processes; however, there are few studies regarding their effects on soil preferential flow in the Mu Us sandy land. An evaluation of the effects of zokor disturbance on their habitat and soil water is important for understanding the ecological role of zokors in the soil ecosystem of the Mu Us sandy land. A field dye-tracing experiment was conducted in the Gechougou watershed on the southeastern edge of the Mu Us sandy land to investigate the effect of zokor burrowing activity on soil preferential flow characteristics. Our results showed that the density of zokor tunnels was the highest (0.40–0.46 m m^-2) under 30%–50% vegetation coverage and that the tunnels were approximately 3 cm from the surface. Both stained area ratio and stained path number were higher at sites with zokors than without zokors. Stained path widths were 10–80 and > 80 mm at zokor-harboring sites exhibiting homogeneous flow and heterogeneous finger flow, respectively. In the absence of zokors, homogeneous flow and highly interacted macropore flow were predominant. Soil water content below the zokor tunnels was higher than that above the tunnels. Moderate disturbance of soil structure by zokor activity facilitated soil water infiltration. These results enabled a better understanding of the effect of soil fauna on soil structure and hydrological processes and provided recommendations for ecological construction and renovation in arid and semi-arid regions.     

Net ecosystem carbon exchange for Bermuda grass growing in mesocosms as affected by irrigation frequency

Intensification of grazed grasslands following conversion from dryland to irrigated farming has the potential to alter ecosystem carbon (C) cycling and affect components of carbon dioxide (CO₂) exchange that could lead to either net accumulation or loss of soil C. While there are many studies on the effect of water availability on biomass production and soil C stocks, much less is known about the effect of the frequency of water inputs on the components of CO₂ exchange. We grew Bermuda grass (Cynodon dactylon L.) in mesocosms under irrigation frequencies of every day (I₁ treatment, 30 d), every two days (I₂ treatment, 12 d), every three days (I₃ treatment, 30 d), and every six days (I₆ treatment, 18 d, after I₂ treatment). Rates of CO₂ exchange for estimating net ecosystem CO₂ exchange (F_N), ecosystem respiration (R_E), and soil respiration (R_S) were measured, and gross C uptake by plants (F_G) and respiration from leaves (R_L) were calculated during two periods, 1–12 and 13–30 d, of the 30-d experiment. During the first 12 d, there were no significant differences in cumulative F_N (mean ±standard deviation, 61 ±30 g C m^-2, n = 4). During the subsequent 18 d, cumulative F_N decreased with decreasing irrigation frequency and increasing cumulative soil water deficit (W), with values of 70 ±22, 60 ±16, and 18 ±12 g C m^-2 for the I₁, I₃, and I₆ treatments, respectively. There were similar decreases in F_G, R_E, and R_L with increasing W, but differences in R_S were not significant. Use of the C₄ grass growing in a C₃-derived soil enabled partitioning of R_S into its autotrophic (R_A) and heterotrophic (R_H) components using a ¹³C natural abundance isotopic technique at the end of the experiment when differences in cumulative W between the treatments were the greatest. The values of R_H and its percentage contributions to R_S (43% ±8%, 42% ±8%, and 8% ±5% for the I₁, I₃, and I₆ treatments, respectively) suggested that R_H remained unaffected across a wide range of W and then decreased under extreme W. There were no significant differences in aboveground biomass between the treatments. Nitrous oxide (N₂O) emission was measured to determine if there was a trade-off effect between irrigation frequency and increasing W on net greenhouse gas emission, but no significant differences were found between the treatments. These findings suggest that over short periods in well-drained soil, irrigation frequency could be managed to manipulate soil water deficit in order to reduce net belowground respiratory C losses, particularly those from the microbial decomposition of soil organic matter, with no significant effect on biomass production and N₂O emission.     

Patterns and drivers of seasonal water sources for artificial sand-fixing plants in the northeastern Mu Us sandy land, Northwest China

Understanding plant water-use patterns is important for improving water-use efficiency and for sustainable vegetation restoration in arid and semi-arid regions. However, seasonal variations in water sources and their control by different sand-fixing plants in water-limited desert ecosystems remain poorly understood. In this study, stable isotopic ratios of hydrogen (δ²H) and oxygen (δ¹⁸O) in precipitation, soil water, groundwater, and xylem water were determined to document seasonal changes in water uptake by three representative plant species (Pinus sylvestris var. mongolica Litv., Amygdalus pedunculata Pall., and Salix psammophila) in the northeastern Mu Us sandy land, Northwest China. Based on the depth distribution and temporal variation of measured gravimetric soil water content (SWC), the soil water profile of the three species stands was divided into active (0.01 g g^-1 < SWC < 0.08 g g^-1, 20% < coefficient of variation (CV) < 45%), stable (0.02 g g^-1 < SWC < 0.05 g g^-1, CV < 20%), and moist (0.08 g g^-1 < SWC < 0.20 g g^-1, CV > 45%) layers. Annually, P. sylvestris, A. pedunculata, and S. psammophila obtained most water from deep (59.2% ±9.7%, moist layer and groundwater), intermediate (57.4% ±9.8%, stable and moist layers), and shallow (54.4% ±10.5%, active and stable layers) sources, respectively. Seasonally, the three plant species absorbed more than 60% of their total water uptake from the moist layer and groundwater in the early (June) dry season; then, they switched to the active and stable layers in the rainy season (July–September) for water resources (50.1%–62.5%). In the late (October–November) dry season, P. sylvestris (54.5%–66.2%) and A. pedunculata (52.9%–63.6%) mainly used water from stable and moist layers, whereas S. psammophila (52.6%–70.7%) still extracted water predominantly from active and stable layers. Variations in the soil water profile induced by seasonal fluctuations in precipitation and groundwater levels and discrepancies in plant phenology, root distribution, and water demand are the main factors affecting the seasonal water-use patterns of artificial sand-fixing plants. Our study addresses the issue of plant water uptake with knowledge of proportional source-water use and reveals important implications for future vegetation restoration and water management in the Mu Us sandy land and similar desert regions around the world.     

伴随阴离子对马铃薯种植冲击土中钾素固持与淋溶的影响

A column study was carried out to assess the influence of accompanying anions on potassium (K) leaching at potato growing sites with different soil textures (sandy loam and clay loam) in northwestern India. Potassium was applied in the top 15 cm layer of soil column at 30 and 60 mg K kg^-1 through different sources having different accompanying anions (Cl^-, SO₄^2-, NO₃^- and H₂PO₄^-). Maximum K was retained in the top 0--15 cm layer with a sharp decrease in K content occurring in 15--30 cm layer of the soil column. The trend was similar for both levels of applied K as well as frequency of leaching and soil type. The decrease of K content in soil column after four leaching events was maximum in case of Khanaura sandy loam, while only minor decrease was observed in Hundowal clay loam when K was applied at 60 mg K kg^-1, indicating higher potential of clay rich soil to adsorb K. In general, the K leaching in presence of the accompanying anions followed the order of SO₄^2- ≤ H₂PO₄^2- < NO₃^- = Cl^-. Highest 1 mol L^-1 CH3COONH4-extractable K was retained when K was applied along with SO42- and H2PO4- anions, and the least was retained when accompanying anion was Cl^-1. The influence of anions was more pronounced in the light textured soil and at high amounts of K application. Higher levels of K application resulted in higher losses of K, especially in sandy loam soil as observed from the leachate concentration. Among the different K sources, the maximum amount of K leaching was noticed in the soil column amended with KCl. After four leachings, the maximum amount of K leached out was 6.40 mg L^-1 in Hundowal clay loam and 9.29 mg L^-1 in Khanaura sandy loam at 60 mg K kg^-1 of soil application through KCl. These concentrations were lower than the recommended guideline of the World Health Organisation (12.00 mg L^-1).     

Vegetation cover and rain timing co-regulate the responses of soil CO2 efflux to rain increase in an arid desert ecosystem

Climate models often predict that more extreme precipitation events will occur in arid and semiarid regions, where C cycling is particularly sensitive to the amount and seasonal distribution of precipitation. Although the effects of precipitation change on soil carbon processes in desert have been studied intensively, how vegetation cover and rain timing co-regulate the responses of soil CO₂ efflux to precipitation change is still not well understood. In this study, a field manipulative experiment was conducted with five simulated rain addition treatments (natural rains plus 0%, 25%, 50%, 75%, 100% of local annual mean precipitation) in a desert ecosystem in Northwest China. The rain addition treatments were applied with 16 field rain enrichment systems on the 10th day of each month from May to September, 2009. Soil water content, soil temperature and soil CO₂ efflux rates were measured in both bare and vegetated soils before and after the rain addition during a 3-week period for each rain treatment. The response magnitude and duration of soil CO₂ efflux to rain addition depended not only on the rain amount but also on the type of vegetation covers and the timing of rain addition treatments. Soil water content responded quickly to the rain addition regardless of rain amount and timing, but soil CO₂ efflux increased to rain addition only in May–July but not in late growing season (September). In addition, soil CO₂ efflux from the bare and vegetated soils showed similar increase to rain additions in May–July, but they demonstrated distinct responses to rain addition in September. The differences in the responses of soil CO₂ efflux to rain addition between the bare and vegetated soils could be explained by the root activities stimulated by added rain water, while the difference in soil CO₂ efflux response to rain addition among treatment times could be attributed to soil water condition prior to rain addition and/or soil temperature drop following rain addition. Thus, both vegetation cover and rain timing can co-regulate responses of soil CO₂ efflux to future precipitation change in arid desert ecosystems, which should be considered when predicting future carbon balance of desert ecosystems in arid and semiarid regions.     

Effects of simulated nitrogen deposition on soil respiration components and their temperature sensitivities in a semiarid grassland

Nitrogen (N) deposition to semiarid ecosystems is increasing globally, yet few studies have investigated the ecological consequences of N enrichment in these ecosystems. Furthermore, soil CO₂ flux – including plant root and microbial respiration – is a key feedback to ecosystem carbon (C) cycling that links ecosystem processes to climate, yet few studies have investigated the effects of N enrichment on belowground processes in water-limited ecosystems. In this study, we conducted two-level N addition experiments to investigate the effects of N enrichment on microbial and root respiration in a grassland ecosystem on the Loess Plateau in northwestern China. Two years of high N additions (9.2 g N m⁻² y⁻¹) significantly increased soil CO₂ flux, including both microbial and root respiration, particularly during the warm growing season. Low N additions (2.3 g N m⁻² y⁻¹) increased microbial respiration during the growing season only, but had no significant effects on root respiration. The annual temperature coefficients (Q₁₀) of soil respiration and microbial respiration ranged from 1.86 to 3.00 and 1.86 to 2.72 respectively, and there was a significant decrease in Q₁₀ between the control and the N treatments during the non-growing season but no difference was found during the growing season. Following nitrogen additions, elevated rates of root respiration were significantly and positively related to root N concentrations and biomass, while elevated rates of microbial respiration were related to soil microbial biomass C (SMBC). The microbial respiration tended to respond more sensitively to N addition, while the root respiration did not have similar response. The different mechanisms of N addition impacts on soil respiration and its components and their sensitivity to temperature identified in this study may facilitate the simulation and prediction of C cycling and storage in semiarid grasslands under future scenarios of global change.     

Impact of land degradation on soil respiration in a steppe (Stipa tenacissima L.) semi-arid ecosystem in the SE of Spain

Climate change scenarios predict increases in temperature, changes in precipitation patterns, and longer drought periods in most semi-arid regions of the world. Ecosystems in these regions are prone to land degradation, which may be aggravated by climate change. Soil respiration is one of the main processes responsible for organic carbon losses from arid and semi-arid ecosystems. We measured soil respiration over one year in two steppe ecosystems having different degrees of land degradation under three ground-covers: with vegetation, bare soil, and an intermediate situation between plants and bare soil.The largest differences in soil respiration rates between the sites were observed in spring, coinciding with the highest level of plant activity. The degraded site had drier and hotter soils with less soil water availability and a longer drought period. As a result, vegetation on the degraded site did not respond to spring rainfall events. Soil respiration showed a strong seasonal variability, with average annual rates of 1.1 and 0.8 μmol CO₂ m⁻² s⁻¹ in the natural and degraded sites, respectively. We did not observe significant differences in soil respiration rates associated with ground-cover i.e., the temporal variation was much larger than the spatial variation. At both sites, soil moisture was the controlling driver of soil respiration for most of the year, when temperatures were above 20 °C and constrained the response to temperature for the few months when the temperature was below 20 °C. An empirical model based on soil temperature and soil moisture explained 90% and 72% of the seasonal variability of soil respiration on the natural and degraded sites, respectively. For the first time, this study suggests that land degradation may alter the carbon balance of these ecosystems through changes in the temporal dynamics of soil respiration and plant productivity, which have important negative consequences for ecosystem functioning and sustainability.     

Temperature dependence of soil CO2 efflux is strongly modulated by seasonal patterns of moisture availability in a Mediterranean ecosystem

Extensive research has focused on the temperature sensitivity of soil respiration. However, in Mediterranean ecosystems, soil respiration may have a pulsed response to precipitation events, especially during prolonged dry periods. Here, we investigate temporal variations in soil respiration (R_s), soil temperature (T) and soil water content (SWC) under three different land uses (a forest area, an abandoned agricultural field and a rainfed olive grove) in a dry Mediterranean area of southeast Spain, and evaluate the relative importance of soil temperature and water content as predictors of R_s. We hypothesize that soil moisture content, rather than soil temperature, becomes the major factor controlling CO₂ efflux rates in this Mediterranean ecosystem during the summer dry season. Soil CO₂ efflux was measured monthly between January 2006 and December 2007 using a portable soil respiration instrument fitted with a soil respiration chamber (LI-6400-09). Mean annual soil respiration rates were 2.06 ± 0.07, 1.71 ± 0.09, and 1.12 ± 0.12 μmol m⁻² s⁻¹ in the forest, abandoned field and olive grove, respectively. R_s was largely controlled by soil temperature above a soil water content threshold value of 10% at 0-15 cm depth for forest and olive grove, and 15% for abandoned field. However, below those thresholds R_s was controlled by soil moisture. Exponential and linear models adequately described R_s responses to environmental variables during the growing and dry seasons. Models combining abiotic (soil temperature and soil rewetting index) and biotic factors (above-ground biomass index and/or distance from the nearest tree) explained between 39 and 73% of the temporal variability of R_s in the forest and olive grove. However, in the abandoned field, a single variable - either soil temperature (growing season) or rewetting index (dry season) - was sufficient to explain between 51 and 63% of the soil CO₂ efflux. The fact that the rewetting index, rather than soil water content, became the major factor controlling soil CO₂ efflux rates during the prolonged summer drought emphasizes the need to quantify the effects of rain pulses in estimates of net annual carbon fluxes from soil in Mediterranean ecosystems.     

Sources of CO2 efflux from soil and review of partitioning methods

Five main biogenic sources of CO₂ efflux from soils have been distinguished and described according to their turnover rates and the mean residence time of carbon. They are root respiration, rhizomicrobial respiration, decomposition of plant residues, the priming effect induced by root exudation or by addition of plant residues, and basal respiration by microbial decomposition of soil organic matter (SOM). These sources can be grouped in several combinations to summarize CO₂ efflux from the soil including: root-derived CO₂, plant-derived CO₂, SOM-derived CO₂, rhizosphere respiration, heterotrophic microbial respiration (respiration by heterotrophs), and respiration by autotrophs. These distinctions are important because without separation of SOM-derived CO₂ from plant-derived CO₂, measurements of total soil respiration have very limited value for evaluation of the soil as a source or sink of atmospheric CO₂ and for interpreting the sources of CO₂ and the fate of carbon within soils and ecosystems. Additionally, the processes linked to the five sources of CO₂ efflux from soil have various responses to environmental variables and consequently to global warming. This review describes the basic principles and assumptions of the following methods which allow SOM-derived and root-derived CO₂ efflux to be separated under laboratory and field conditions: root exclusion techniques, shading and clipping, tree girdling, regression, component integration, excised roots and insitu root respiration; continuous and pulse labeling, ¹³C natural abundance and FACE, and radiocarbon dating and bomb-¹⁴C. A short sections cover the separation of the respiration of autotrophs and that of heterotrophs, i.e. the separation of actual root respiration from microbial respiration, as well as methods allowing the amount of CO₂ evolved by decomposition of plant residues and by priming effects to be estimated. All these methods have been evaluated according to their inherent disturbance of the ecosystem and C fluxes, and their versatility under various conditions. The shortfalls of existing approaches and the need for further development and standardization of methods are highlighted.     

Soil Respiration of Biologically-Crusted Soils in Response to Simulated Precipitation Pulses in the Tengger Desert, Northern China

Soil respiration (SR) is a major process of carbon loss from dryland soils, and it is closely linked to precipitation which often occurs as a discrete episodic event. However, knowledge on the dynamic patterns of SR of biologically-crusted soils in response to precipitation pulses remains limited. In this study, we investigated CO₂ emissions from a moss-crusted soil (MCS) and a cyanobacterialichen-crusted soil (CLCS) after 2, 4, 8, 16, and 32 mm precipitation during the dry season in the Tengger Desert, northern China. Results showed that 2 h after precipitation, the SR rates of both MCS and CLCS increased up to 18-fold compared with those before rewetting, and then gradually declined to background levels; the decrease was faster at lower precipitation amount and slower at higher precipitation amount. The peak and average SR rates over the first 2 h in MCS increased with increasing precipitation amount, but did not vary in CLCS. Total CO₂ emission during the experiment (72 h) ranged from 1.35 to 5.67 g C m-² in MCS, and from 1.11 to 3.19 g C m-² in CLCS. Peak and average SR rates, as well as total carbon loss, were greater in MCS than in CLCS. Soil respiration rates of both MCS and CLCS were logarithmically correlated with gravimetric soil water content. Comparisons of SR among different precipitation events, together with the analysis of long-term precipitation data, suggest that small-size precipitation events have the potential for large short-term carbon losses, and that biological soil crusts might significantly contribute to soil CO₂ emission in the water-limited desert ecosystem.     

Winter soil CO2 efflux and its contribution to annual soil respiration in different ecosystems of a forest-steppe ecotone, north China

Most soil respiration measurements are conducted during the growing season. In tundra and boreal forest ecosystems, cumulative winter soil CO₂ fluxes are reported to be a significant component of their annual carbon budgets. However, little information on winter soil CO₂ efflux is known from mid-latitude ecosystems. Therefore, comparing measurements of soil respiration taken annually versus during the growing season will improve the accuracy of ecosystem carbon budgets and the response of soil CO₂ efflux to climate changes. In this study we measured winter soil CO₂ efflux and its contribution to annual soil respiration for seven ecosystems (three forests: Pinus sylvestris var. mongolica plantation, Larix principis-rupprechtii plantation and Betula platyphylla forest; two shrubs: Rosa bella and Malus baccata; and two meadow grasslands) in a forest-steppe ecotone, north China. Overall mean winter and growing season soil CO₂ effluxes were 0.15-0.26 μmol m⁻² s⁻¹ and 2.65-4.61 μmol m⁻² s⁻¹, respectively, with significant differences in the growing season among the different ecosystems. Annual Q₁₀ (increased soil respiration rate per 10 °C increase in temperature) was generally higher than the growing season Q₁₀. Soil water content accounted for 84% of the variations in growing season Q₁₀ and soil temperature range explained 88% of the variation in annual Q₁₀. Soil organic carbon density to 30 cm depth was a good surrogate for SR₁₀ (basal soil respiration at a reference temperature of 10 °C). Annual soil CO₂ efflux ranged from 394.76 g C m⁻² to 973.18 g C m⁻² using observed ecosystem-specific response equations between soil respiration and soil temperature. Estimates ranged from 424.90 g C m⁻² to 784.73 g C m⁻² by interpolating measured soil respiration between sampling dates for every day of the year and then computing the sum to obtain the annual value. The contributions of winter soil CO₂ efflux to annual soil respiration were 3.48-7.30% and 4.92-7.83% using interpolated and modeled methods, respectively. Our results indicate that in mid-latitude ecosystems, soil CO₂ efflux continues throughout the winter and winter soil respiration is an important component of annual CO₂ efflux.     

Strong seasonal variations in net ecosystem CO2 exchange of a tropical pasture and afforestation in Panama

Pasture and afforestation are land-use types of major importance in the tropics, yet, most flux tower studies have been conducted in mature tropical forests. As deforestation in the tropics is expected to continue, it is critical to improve our understanding of alternative land-use types, and the impact of interactions between land use and climate on ecosystem carbon dynamics. Thus, we measured net ecosystem CO₂ fluxes of a pasture and an adjacent tropical afforestation (native tree species plantation) in Sardinilla, Panama from 2007 to 2009. The objectives of our paired site study were: (1) to assess seasonal and inter-annual variations in net ecosystem CO₂ exchange (NEE) of pasture and afforestation, (2) to identify the environmental controls of net ecosystem CO₂ fluxes, and (3) to constrain eddy covariance derived total ecosystem respiration (TER) with chamber-based soil respiration (R_Soil) measurements. We observed distinct seasonal variations in NEE that were more pronounced in the pasture compared to the afforestation, reflecting changes in plant and microbial activities. The land conversion from pasture to afforestation increased the potential for carbon uptake by trees vs. grasses throughout most of the year. R_Soil contributed about 50% to TER, with only small differences between ecosystems or seasons. Radiation and soil moisture were the main environmental controls of CO₂ fluxes while temperature had no effect on NEE. The pasture ecosystem was more strongly affected by soil water limitations during the dry season, probably due to the shallower root system of grasses compared to trees. Thus, it seems likely that predicted increases in precipitation variability will impact seasonal variations of CO₂ fluxes in Central Panama, in particular of pasture ecosystems.     

Estimating transpiration and the sensitivity of carbon uptake to water availability in a subalpine forest using a simple ecosystem process model informed by measured net CO2 and H2O fluxes

Modeling how the role of forests in the carbon cycle will respond to predicted changes in water availability hinges on an understanding of the processes controlling water use in ecosystems. Recent studies in forest ecosystem modeling have employed data-assimilation techniques to generate parameter sets that conform to observations, and predict net ecosystem CO₂ exchange (NEE) and its component processes. Since the carbon and water cycles are linked, there should be additional process information available from ecosystem H₂O exchange. We coupled SIPNET (Simple Photosynthesis EvapoTranspiration), a simplified model of ecosystem function, with a data-assimilation system to estimate parameters leading to model predictions most closely matching the net CO₂ and H₂O fluxes measured by eddy covariance in a high-elevation, subalpine forest ecosystem. When optimized using measurements of CO₂ exchange, the model matched observed NEE (RMSE = 0.49 g C m⁻²) but underestimated transpiration calculated independently from sap flow measurements by a factor of 4. Consequently, the carbon-only optimization was insensitive to imposed changes in water availability. Including eddy flux data from both CO₂ and H₂O exchange to the optimization reduced the model fit to the observed NEE fluxes only slightly (RME = 0.53 g C m⁻²), however this parameterization also reproduced transpiration calculated from independent sap flow measurements (r² = 0.67, slope = 0.6). A significant amount of information can be extracted from simultaneous analysis of CO₂ and H₂O exchange, which improved the accuracy of transpiration estimates from measured evapotranspiration. Conversely, failure to include both CO₂ and H₂O data streams can generate results that mask the responses of ecosystem carbon cycling to variation in the precipitation. In applying the model conditioned on both CO₂ and H₂O fluxes to the subalpine forest at the Niwot Ridge AmeriFlux site, we observed that the onset of transpiration is coincident with warm soil temperatures. However, after snow has covered the ground in the fall, we observed significant inter-annual variability in the fraction of evapotranspiration composed of transpiration; evapotranspiration was dominated by transpiration in years when late fall air temperatures were high enough to maintain photosynthesis, but by sublimation from the surface of the snowpack in years when late fall air temperatures were colder and forest photosynthetic activity had ceased. Data-assimilation techniques and simultaneous measurements of carbon and water exchange can be used to quantify the response of net carbon uptake to changes in water availability by using an ecosystem model where the carbon and water cycles are linked.     

Hot spots,hot moments,and spatio-temporal controls on soil CO2 efflux in a water-limited ecosystem

Soil CO₂ efflux is the primary source of CO₂ emissions from terrestrial ecosystems to the atmosphere. The rates of this flux vary in time and space producing hot moments (sudden temporal high fluxes) and hot spots (spatially defined high fluxes), but these high reaction rates are rarely studied in conjunction with each other. We studied temporal and spatial variation of soil CO₂ efflux in a water-limited Mediterranean ecosystem in Baja California, Mexico. Soil CO₂ efflux increased 522% during a hot moment after rewetting of soils following dry summer months. Monthly precipitation was the primary driver of the seasonal trend of soil CO₂ efflux (including the hot moment) and through changes in soil volumetric water content (VWC) it influenced the relationship between CO₂ efflux and soil temperature. Geostatistical analyses showed that the spatial dependence of soil CO₂ efflux changed between two contrasting seasons (dry and wet). During the dry season high soil VWC was associated with high soil CO₂ efflux, and during the wet season the emergence of a hot spot of soil CO₂ efflux was associated with higher root biomass and leaf area index. These results suggest that sampling designs should accommodate for changes in spatial dependence of measured variables. The spatio-temporal relationships identified in this study are arguably different from temperate ecosystems where the majority of soil CO₂ efflux research has been done. This study provides evidence of the complexity of the mechanisms controlling the spatio-temporal variability of soil CO₂ efflux in water-limited ecosystems.